Fix a SEGV with 3rd-party FS module loading when svn_fs_initialize
is called a second time after clearing the pool passed to the first
call.

* subversion/libsvn_fs/fs-loader.c
  (uninit): Reset linked list when common pool is cleared.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1519615 13f79535-47bb-0310-9956-ffa450edef68